源码:
initialize

// **************** org.springframework.boot.SpringApplication ******************/
private List<ApplicationContextInitializer<?>> initializers;

private void initialize(Object[] sources) {
  //略...
  setInitializers((Collection) getSpringFactoriesInstances(
      ApplicationContextInitializer.class));
  //略...
}
private List<ApplicationContextInitializer<?>> initializers;
public void setInitializers(
      Collection<? extends ApplicationContextInitializer<?>> initializers) {
  this.initializers = new ArrayList<ApplicationContextInitializer<?>>();
  this.initializers.addAll(initializers);
}

（1）创建所有 classpath 下所有 “Initializer”对象

源码:

initialize

 |- getSpringFactoriesInstances


// **************** org.springframework.boot.SpringApplication ******************/
private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) {
  return getSpringFactoriesInstances(type, new Class<?>[] {});
}
private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
    Class<?>[] parameterTypes, Object... args) {
  ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
  // Use names and ensure unique to protect against duplicates
  Set<String> names = new LinkedHashSet<String>(
      SpringFactoriesLoader.loadFactoryNames(type, classLoader));
  List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
      classLoader, args, names);
  AnnotationAwareOrderComparator.sort(instances);
  return instances;
}

（1.1）获取 classpath 下所有 “Initializer” 的全类名

源码:

initialize

 |- getSpringFactoriesInstances

     |- SpringFactoriesLoader.loadFactoryNames


​​SpringFactoriesLoader.loadFactoryNames（）​​ 方法在classpath下遍历 “META-INF/spring.factories” 文件，最终将这个文件中所有factoryClass指定类型的全类名解析出来，放入ArrayList中作为返回值

// **************** org.springframework.core.io.support.SpringFactoriesLoader ******************/
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
  String factoryClassName = factoryClass.getName();
  try {
    Enumeration<URL> urls 
      = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
        ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
    List<String> result = new ArrayList<String>();
    while (urls.hasMoreElements()) {
      //本例中，其中一个spring.factories为：
      //jar:file:/D:/../org/springframework/boot/spring-boot/1.5.9.RELEASE/
      //spring-boot-1.5.9.RELEASE.jar!/META-INF/spring.factories
      URL url = urls.nextElement();
      // 将文件解析为Properties 类型
      Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
      // 从Properties文件中获取key为 factoryClassName 的值
      String factoryClassNames = properties.getProperty(factoryClassName);
      // 逗号分隔的字符串解析为String[],然后转为List放入result
      result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
    }
    return result;
  }
  catch (IOException ex) {
    throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
        "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
  }
}

返回结果：
names = {LinkedHashSet@1647}  size = 6
 0 = "o.s.b.context.ConfigurationWarningsApplicationContextInitializer"
 1 = "o.s.b.context.ContextIdApplicationContextInitializer"
 2 = "o.s.b.context.config.DelegatingApplicationContextInitializer"
 3 = "o.s.b.context.embedded.ServerPortInfoApplicationContextInitializer"
 4 = "o.s.b.autoconfigure.SharedMetadataReaderFactoryContextInitializer"
 5 = "o.s.b.autoconfigure.logging.AutoConfigurationReportLoggingInitializer"

🤔 Spring Boot 为什么把这些类配置在classpath下的 “META-INF/spring.factories” 文件中？

其实这是个扩展机制，用户自己可以开发一些第三方应用，在应用中创建 “META-INF/spring.factories” 文件，并在里面设置一些 Initializer ，用户在 Initializer 里实现自己的逻辑，这样在Spring Boot启动时就可以执行这些逻辑了。

（1.2）创建所有 “Initializer”实例

前面已经获取了classpath下所有的 Initializer 了，现在将其作为参数，调用 ​​createSpringFactoriesInstances(...)​​ 方法来创建这些 Initializer 的对象，并将创建好的对象放入 SpringApplication 中。

源码:

initialize

 |- getSpringFactoriesInstances

     |- createSpringFactoriesInstances 


// **************** org.springframework.boot.SpringApplication ******************/
@SuppressWarnings("unchecked")
private <T> List<T> createSpringFactoriesInstances(Class<T> type,
    Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args,
    Set<String> names) {
  List<T> instances = new ArrayList<T>(names.size());
  for (String name : names) {
    try {
      Class<?> instanceClass = ClassUtils.forName(name, classLoader);
      Assert.isAssignable(type, instanceClass);
      Constructor<?> constructor = instanceClass
          .getDeclaredConstructor(parameterTypes);
      // 通过反射实例化对象
      T instance = (T) BeanUtils.instantiateClass(constructor, args);
      // 通过反射实例化对象
      instances.add(instance);
    }
    catch (Throwable ex) {
      throw new IllegalArgumentException(
          "Cannot instantiate " + type + " : " + name, ex);
    }
  }
  return instances;
}

（2）设置 SpringApplication 的“Initializer”

源码:

initialize

 |- setInitializers


// **************** org.springframework.boot.SpringApplication ******************/
private List<ApplicationContextInitializer<?>> initializers;
public void setInitializers(
      Collection<? extends ApplicationContextInitializer<?>> initializers) {
  this.initializers = new ArrayList<ApplicationContextInitializer<?>>();
  this.initializers.addAll(initializers);
}

1.2-Q1 什么是 ApplicationContextInitializer ？什么场景下使用？

1.2-Q2 设置这个initializers做什么用？

0 = "o.s.b.context.ConfigurationWarningsApplicationContextInitializer"
 1 = "o.s.b.context.ContextIdApplicationContextInitializer"
 2 = "o.s.b.context.config.DelegatingApplicationContextInitializer"
 3 = "o.s.b.context.embedded.ServerPortInfoApplicationContextInitializer"
 4 = "o.s.b.autoconfigure.SharedMetadataReaderFactoryContextInitializer"
 5 = "o.s.b.autoconfigure.logging.AutoConfigurationReportLoggingInitializer"

1、搜索这个几个类，查看 ​​initialize​​​方法做了哪些事情？
2、选一个类，在其 ​​​initialize​​ 第一行打上断点，debug程序，看看方法时候调用的？